210 C. D. SMITH AND L. A. COTTON. 



The letters indicate the names proposed for the forms. 

 These are the same as the letters given to similar faces on 

 stolzite and wulfenite where these corresponding forms 

 are known. 



The indices are given in Miller's notation. The measured 

 angles for <£ have been placed in two columns which are so 

 arranged that the first order pyramids in either column 

 are in the same zone. The signs + and - prefixed to the 

 <j> reading indicate faces developed on the same and opposite 

 sides of the crystal respectively, as the standard basal 

 plane. Faces underlined in the Tables VI and VII indicate 

 faces in the same zone as but along the opposite edge to the 

 other faces in the same column. 



Paces marked with an asterisk are referred to the forms 

 represented by the letters by which they are distinguished, 

 but were not considered to be sufficiently well developed 

 to be included in deducing the mean values represented in 

 Table VIII. The columns marked E</> and E/> show the 

 difference in minutes from the mean values of the gonio- 

 meter measurements. These have been incorporated as a 

 measure of the degree of perfection of the crystal faces. 

 The columns <ty and §p express the differences of the 

 measured </> and p from the theoretical value calculated 

 from the corresponding indices. 



On all the crystals measured two basal planes were 

 present, one of which showed a relatively good signal and 

 the other a very poor one. 



The latter was in some cases only represented by an 

 indistinct blur of light. The basal plane with the better 

 signal was in each case selected as the standard of refer- 

 ence for the other faces. 



Crystal No. 1. — The forms developed are c, f and e. Of 

 these the dominant ones are c and f . The faces approxi- 

 mating to the form y were too small to be seen, but their 

 reflections show that they were symmetrically developed. 



